plugin loading failure should be considered an error so build will not try to run

hcl2template/types.build.post-processor.go

@@ -59,6 +59,7 @@ func (cfg *PackerConfig) startPostProcessor(source SourceUseBlock, pp *PostProce
 	postProcessor, err := cfg.parser.PluginConfig.PostProcessors.Start(pp.PType)
 	if err != nil {
 		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
 			Summary:  fmt.Sprintf("Failed loading %s", pp.PType),
 			Subject:  pp.DefRange.Ptr(),
 			Detail:   err.Error(),

hcl2template/types.build.provisioners.go

@@ -146,6 +146,7 @@ func (cfg *PackerConfig) startProvisioner(source SourceUseBlock, pb *Provisioner
 	provisioner, err := cfg.parser.PluginConfig.Provisioners.Start(pb.PType)
 	if err != nil {
 		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
 			Summary:  fmt.Sprintf("failed loading %s", pb.PType),
 			Subject:  pb.HCL2Ref.LabelsRanges[0].Ptr(),
 			Detail:   err.Error(),

hcl2template/types.source.go

@@ -101,6 +101,7 @@ func (cfg *PackerConfig) startBuilder(source SourceUseBlock, ectx *hcl.EvalConte
 	builder, err := cfg.parser.PluginConfig.Builders.Start(source.Type)
 	if err != nil {
 		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
 			Summary:  "Failed to load " + sourceLabel + " type",
 			Detail:   err.Error(),
 		})